The differences in the effects of various diets are significant but small

Action Points

Low-fat diets did not lead to long-term weight loss in a large meta-analysis of randomized controlled trials.

The study suggests that additional research is needed to identify optimum intervention strategies for long-term weight loss and weight maintenance.

Low-fat diets did not lead to long-term weight loss in a large meta-analysis of randomized controlled trials.

Researchers looked at 53 studies with more than 68,000 participants, each study lasting at least 1 year. They found that low-carbohydrate diets led to small but significantly greater weight loss than did low-fat interventions in 18 of the comparisons (weighted mean difference (WMD) 1.15 kg, 95% CI 0.52-1.79; I2=10%).

In addition, low-fat diets didn't outperform higher-fat diets when it came to weight loss (WMD 0.36 kg, 95% CI -0.66 to 1.37; I2=82%), but they did lead to greater weight loss when compared with usual diets in eight comparisons (WMD -5.41 kg, 95% CI -7.29 to -3.54, I2=68%), according to author Frank Hu, MD, PhD, at the Harvard TH Chan School of Public Health, and colleagues.

"Health and nutrition guidelines should cease recommending low-fat diets for weight loss in view of the clear absence of long-term efficacy when compared with other similar intensity dietary interventions," wrote the authors. "Additional research is needed to identify optimum intervention strategies for long-term weight loss and weight maintenance, including the need to look beyond variations in macronutrient composition."

According to Robert Eckel, MD, at the University of Colorado Denver, the difference of around 1 kg of weight loss between low-carb and low-fat diets is "clinically meaningless," adding that "weight loss is caloric restriction."

In an accompanying editorial, Kevin Hall, PhD, at the National Institutes of Diabetes and Digestive and Kidney Diseases, agreed that the difference that the researchers found between diets may not mean much.

"Before proclaiming the superiority of low-carbohydrate diets for the treatment of obesity, consider the magnitude of the benefit: participants prescribed low-carbohydrate diets lost only about 1 kg of additional weight after 1 year compared with those advised to consume low-fat diets," wrote Hall. "Irrespective of the diet prescription, the overall average weight loss in trials testing interventions designed to reduce body weight was unimpressive."

Twenty of the trials included specifically enrolled patients with chronic diseases and 35 were designed specifically to compare weight loss interventions; 13 of the trials had no intended intervention for weight loss, and five were designed to maintain a certain body weight. The dietary interventions ranged from very low, where ≤10% of calories were from fat, to more moderate, with ≤30% of calories from fat.

How aggressive the intervention was also varied by study. Some interventions included pamphlets or instructions given at baseline only, while others were feeding studies in which participants were given food. Some of the studies attempted to restrict calories as well.

Hall added that it's important to consider adherence when looking at the findings, which he says probably lapsed long before the one year mark. Other studies have shown that outpatient weight-loss peaks at 6-8 months, followed by weight regain, he noted.

"Energy balance calculations suggest that at the point of maximum weight loss, diet adherence has already substantially waned," he wrote. "Much more research is needed to determine the factors that affect diet adherence and thereby help maintain weight loss over the long term. What seems to be clear is that long-term diet adherence is abysmal, irrespective of whether low-fat or other diets, such as low-carbohydrate diets, are prescribed."

In trials where the intervention was specifically intended to lead to weight loss and when low-fat and higher-fat groups differed by more than 5% of calories obtained from fat, higher fat diets led to greater weight loss (18 comparisons; WMD 1.04 kg, 95% CI 0.06-2.03; I2=78%). The same was true for when the difference in serum triglycerides was at least 0.06 mmol/L (17 comparisons; 1.38 kg, 95% CI 0.50-2.25; I2=62%).

Keith Ayoob, RD, EdD, at the Albert Einstein College of Medicine in New York, added that he didn't find the results surprising, since there are lots of ways to lose weight. "That's not the real challenge. It's a challenge to keep them motivated enough to maintain compliance with a particular regimen after they lose the weight, so they can keep it off," he wrote in an email to MedPage Today. "That's why when a diet is too restrictive -- in any direction, whether it's low-carb or low-fat -- after a while people just begin to lose their mojo with the regimen."

Limitations of the study included significant heterogeneity between the studies and the possibility of missing relevant studies. In addition, most of the studies were not feeding trials, and previous research has shown that data can be limited when they rely on food surveys and other methods.

The study was supported by grants from the NIH.

Tobias was supported by a fellowship from the American Diabetes Association.

Hall disclosed being supported by the NIH and has a patent pending related to personalized dynamic feedback control of bodyweight.

Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco

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